Gallium arsenide substrate wafers are used for the production of high-frequency amplifiers and switches as well as light-emitting components such as semiconductor lasers and luminescence diodes. The component structures (transistors and diodes) are typically produced from epitaxially deposited mixed crystal stacks of variable elements which can be selected from the group of Ga—In—As—P—Al—N, wherein in the individual layers different electronic properties can be set at same crystal lattice parameters by specific compositions. This kind of lattice-adapted epitaxy aims at a very high layer quality and low defect densities. In this regard the layer quality depends not only on the conditions in the epitaxy process, but it is also influenced by the substrate properties.
The production of GaAs substrates starts with the growing of respective bulk crystals and the subsequent singularization in a sawing/separating process (see for example T. Flade et al., “State of the art 6″ SI GaAs wafers made of conventionally grown LEC-crystals”, Journal of Crystal Growth, 198-199(1), 1999, p. 336-342 and Th. Bünger et al., “Development of a vertical gradient freeze process for low EPD GaAs substrates”, Materials Science and Engineering B, 80(1), 2001, p. 5-9). Subsequently the substrates are treated in a multi-stage process, inter alia polished, in order to obtain advantageous properties of geometry (thickness, curvature, wedge shape) and roughness (see for example Flade et al.). After the last polishing step the pure highly reactive GaAs surface is exposed, and an oxide growth inevitably starts immediately. The typical subsequent cleaning in liquid media entails a sequence of oxide forming and respectively oxide removing steps and primarily serves to decrease the number of particles and of residual impurities or respectively residual contaminations on the substrate. During the last cleaning steps and the subsequent drying of the substrate an oxide layer is formed. This oxide layer can still undergo change during the time until the insertion of the substrates into the epitaxy apparatus (see for example D. A. Allwood, S. Cox, N. J. Mason, R. Palmer, R. Young, P. J. Walker, Thin Solid Films, 412, 2002, p. 76-83).
The composition of the oxide layer can be measured for example by means of X-ray-excited photoelectron spectroscopy (XPS) in which core electrons of the oxidized atom are spectroscopically investigated. The oxidation states can be determined from the energetic shifts of the excited electrons. The measurement methodology is described for the example of GaAs in detail in C. C. Surdu-Bob, S. O. Saied, J. L Sullivan, Applied Surface Science, Volume 183(1-2), 2001, p. 126-136. Depending on the oxidation conditions the ratio of arsenic and gallium oxides lies between 1 and 5. Typically GaO, Ga2O3 and As2O, AsO, As2O3, As2O5 occur (see for example F. Schröder-Oeynhausen, “Oberflächenanalytische Charakterisierung von metallischen Verunreinigungen and Oxiden auf GaAs”, Dissertation, University of Münster, 1996 and J. S. Song, Y. C. Choi, S. H. Seo, D. C. Oh, M. W. Cho, T. Yao, M. H. Oh, Journal of Crystal Growth, 264, 2004, p. 98-103).
In order to influence the growth of epitaxial layers also by the surface properties of the wafer surface, so far the possibility for the later thermal desorption of surface layers (e.g. oxides) immediately before the start of the epitaxial processes in the epitaxy apparatus was frequently used. In this respect the roughness of a thermally desorbed surface, the degree of contaminations and impurities (particles) play a role in the qualitative properties of the layer stacks to be deposited and the components produced therefrom. For III/V semiconductors the dependence of the quality of epitaxially deposited layers on parameters of the processes used for the preceding cleaning and surface properties associated therewith were investigated. In this regard it was putatively presumed that the wet chemical treatment on the whole surface of the wafer is homogenous in each of the steps.
The wet chemical cleaning of wafers is typically performed in successive liquid baths. Process racks with wafers are usually set from bath to bath by means of automatic transport systems and finally dried. In most cases the cleaning comprises a sequence of acidic and alkaline wet steps with intermediate rinsing steps in deionized water (DI water). In this respect ammonium hydroxide (NH4OH) or organic amines are usually used as alkaline components. Acids used are for example hydrogen fluoride (HF) and hydrogen chloride (HCl), further also sulfuric acid (H2SO4) or organic acids. Often the cleaning media additionally contain additives such as for example oxidizing agents, surfactants (surface-active agents) or chelating agents. For the removal of particles for example ultrasound or megasound in individual baths is used. For the drying of wafers rinsed with DI water in principle diverse processes are conceivable, in practice the drying conventionally relies on the removal of the DI water using the centrifugal forces during the rapid rotation of the wafers or respectively wafer carriers (spin drying) (see e.g. Song et al.).
However, the conventional processes do not provide gallium arsenide substrates which fulfill the increasing requirements on subsequent epitaxy processes in that the large-area and reliable epitaxial production of components with the needed layer quality and the required defect densities in appropriate yield is to be enabled.
The object of the present invention is to provide an improved process for the production of gallium arsenide substrates exhibiting favorable properties for a subsequent epitaxy.